The present invention relates to the field of sulfur removal from hydrocarbon fuels, including diesel oil. In particular, the present invention relates to a new catalytic oxidation/extraction process for the removal of sulfur containing compounds from middle distillates.
Hydrocarbon fuels that are presently used to power diesel engines typically comprise 500 ppm of sulfur. In the interests of reducing environmental pollution, there are continuing efforts to generate simpler and more effective methods to reduce the sulfur content of diesel fuels, which may be applied to an industrial scale.
Existing techniques for the removal of sulfur-containing compounds from hydrocarbon fuels have traditionally involved catalytic hydrogenation under pressure. Although such techniques are relatively inexpensive, the concentration of sulfur in the product fuels is typically greater than 500 ppm. Subjecting the fuel to multiple rounds of hydrogenation can achieve lower final sulfur concentrations. However, sulfur-containing compounds that are sterically hindered are not amenable to extraction by such techniques. As a result, even after multiple rounds of hydrogenation, sulfur concentrations of less than 100 ppm are generally unobtainable. Moreover, multiple hydrogenation steps can increase the production costs of the fuels to levels that are not economically viable.
More recently, the development of oxidation techniques has resulted in increased efficiency of sulfur removal from hydrocarbon fuels. Typically, related processes involve two principle steps. In the first step, the sulfur-containing compounds (present in the hydrocarbon fuel) are oxidized for example by oxidants such as peroxy organic acids, catalyzed hydroperoxides, inorganic peroxy acids or peroxy salts. The oxidized compounds generated include sulphoxides or sulphones resulting from oxygen donation to thiol and thiophene groups.
In the second step of the process, the oxidized products (which are more polarized) can be readily extracted from the hydrocarbon fuel using a polar solvent. Typically, the polar solvent may be a lower alcohol such as methanol, which is partially miscible with diesel oil; a property which confers the advantage of ensuring homogeneous distribution of the polar solvent into the hydrocarbon fuel. This ensures maximal exposure of the oxidized compounds to the polar solvent, thus resulting in optimal extraction of sulfur from the fuel. When the mixture is transferred to conditions that induce phase separation, the oxidized sulfur-containing compounds may be drawn off in the methanol phase, leaving behind a hydrocarbon fuel with a reduced sulfur content.
Generally, it is known in the art that the limiting factor governing the efficiency of sulfur removal is the initial oxidation process. The greater percentage of sulfur-containing compounds that are oxidized, the more sulfur may be removed at extraction. For this reason, developments in the field have attempted to improve oxidation efficiency.
For example, U.S. Pat. No. 3,816,301, issued Jun. 11, 1974, teaches a method for the desulfurization of hydrocarbon material involving oxidation of sulfurons compounds via a peroxy-oxidant in the presence of a molybdenum containing catalyst, and at least one saturated alcohol. In this case, the alcohol is preferably tertiary butyl alcohol, which functions to promote sulfur oxidation by reducing the viscosity of the oxidation reaction mass.
U.S. Pat. Nos. 3,945,914 and 3,970,545 issued Mar. 23, 1976 and Jul. 20, 1976 respectively, disclose further improvements to the oxidation/extraction process. U.S. Pat. No. 3,945,914 claims a process involving oxidation of sulfur-containing compounds followed by heating the fuel to a temperature at which the oxidized sulfur-containing compounds are evaporated, and subsequently reacted with a metal, thus separating the sulfur from the hydrocarbon fuel. Preferably, an oxidation catalyst is present, and a tertiary butyl alcohol can be present as a solvent. U.S. Pat. No. 3,970,545 discloses similar methods, wherein prior to oxidation the method further comprises the step of hydrogenating the sulfur-containing hydrocarbon feedstock in a non-catalytic process to form hydrogen sulfide. In the catalytic oxidation step, the catalyst is preferably prepared from molybdenum metal partially dissolved in an alcohol, such as a tertiary butyl alcohol. U.S. Pat. Nos. 3,945,914 and 3,970,545 therefore both disclose the use of alcohol as a solvent for the oxidation catalyst.
Processes involving alternative oxidation conditions have also been developed. For example U.S. Pat. No. 6,160,193, issued Dec. 12, 2000, discloses an oxidation/extraction process, wherein the oxidation process is monitored and stopped before oxidation of hydrocarbon compounds can ensue. The principle improvements of this patent relate specifically to the monitoring of the reaction process to ensure hydrocarbon oxidation does not occur. In preferred features of the invention, the patent teaches that the oxidant may be an acid such as peroxyacetic acid or peroxysulfuric acid. In this way, the liquid phase oxidation does not involve solid catalyst. The patent also teaches that the preferred extraction solvent is dimethylsulfoxide (DMSO), which results in efficient removal of oxidized species. However, it is important to note that the use of DMSO contaminates the hydrocarbon fuel with sulfur. To remove the DMSO from the fuel mixture, multiple water washing steps are required. In summary, U.S. Pat. No. 6,160,193 teaches a long, complex and expensive procedure for sulfur removal from hydrocarbon fuel.
U.S. Pat. No. 6,171,478 discloses a process for desulfurization of a hydrocarbon oil, involving both hydrodesulfurization and oxidation/extraction. The patent teaches that the fuel may be contacted with a hydrodesulfurization catalyst, thus generating hydrogen sulfide and a first hydrocarbonaceous oil stream. Subsequently, the first hydrocarbonaceous oil stream (with reduced sulfur content) is treated with an oxidizing agent (which in one embodiment is aqueous), which is partially decomposed after the oxidation step. The sulfur-oxidated compounds are then separated (using an appropriate solvent as necessary), and the resulting hydrocarbon fuel (with reduced sulfur content) is isolated. In an alternative embodiment, the extraction solvent comprising sulfur-oxidized compounds, may be recycled. Preferred solvents include acetonitrile, dimethyl formamide, and sulpholane, all of which are sources of nitrogen or sulfur. Therefore, these solvents can contaminate the feed stock with additional nitrogenous or sulfurous compounds, and additional purification steps may be needed to ensure complete removal of such compounds from the final fuel product. In summary, U.S. Pat. No. 6,171,478 essentially discloses a combination of processes, which are known in the art, to generate hydrocarbonaceous fuels with reduced sulfur content.
There is a continuing need to generate hydrocarbon fuels comprising ultra-low levels of sulfur content. Importantly, it is desirable that novel methods for sulfur extraction employ a minimal number of steps, to enable facile desulfurization on an industrial scale. It is further desirable to design such desulfurization techniques to utilize non-toxic and inexpensive reagents that are readily amenable to recycling.
It is therefore an object of the present invention to provide a relatively simple method for extracting sulfur-containing compounds from diesel fuels that is applicable for use on an industrial scale. It is further an object of the present invention to provide a process for the efficient oxidation of sulfur compounds present in middle distillates, without the need for acids or other reactive or toxic chemicals (which can contaminate the feed stock). It is a further object of the invention to provide a process for the production of a hydrocarbonaceous fuel with reduced sulfur content, wherein the sulfur-containing compounds are oxidized and extracted using a non-nitrogen and non-sulfur containing solvent, such as methanol. It is a further object of the invention to provide a process for the production of a hydrocarbonaceous fuel comprising less than 50 ppm sulfur.
The present invention discloses a method for the desulfurization of petroleum middle distillates, in which ethanol is present throughout the catalytic oxidation step. In this way, the oxidation catalyst (typically a metal catalyst) is endowed with a dual role. The oxidation catalyst and H2O2 can function directly to induce oxidation of sulfur-containing species. In addition, the catalyst and H2O2 can oxidize a small fraction of ethanol present in the reaction, thus generating the corresponding peracetic acid. In turn, the peracetic acid helps to drive the oxidation of the sulfur-containing compounds by converting thioethers to sulfoxides and sulfones, which remain solublised in the ethanol. Therefore, the presence of ethanol during catalytic oxidation helps to accelerate the oxidation reaction, the ethanol being the precursor of the co-catalyst, peracetic acid. This results in an improved efficiency of sulfur removal upon subsequent extraction with a polar solvent.
The use of ethanol as a catalytic precursor presents additional advantages. Since the ethanol may be partially miscible with diesel oil, homogeneous distribution of the catalytic precursor is achieved throughout the fuel. Moreover, the sulfoxide and sulfone products remain solublized in the alcohol following oxidation. The alcohol containing dissolved sulfoxides and sulfones may form a distinct phase at room temperature, thus permitting a portion of the oxidized compounds to be removed. The remaining alcohol (and remaining sulfoxides and sulfones) may be removed by extraction with a polar solvent, such as methanol.
Optionally, the methods of the present invention may include an additional step of catalytic hydrogenation, to reduce the overall sulfur content of the hydrocarbon fuel, prior to oxidation and extraction.